To achieve higher mechanical strength, mechanical structural components made of cast iron, steel, etc. conventionally undergo surface hardening treatment such as nitriding, nitrocarburizing, carburizing and quenching, and induction hardening. A nitride layer formed on the outermost surface by nitriding, of these treatment methods, a compound layer on the surface side, in particular, is known to have excellent sliding resistance and also have high fatigue strength.
A hardened layer formed by conventional nitriding or nitrocarburizing treatment is thinner than the one formed by carburizing and quenching or induction hardening, and consequently, there is room for improvement regarding surface durability, fatigue strength, etc.
In view of the above situation, a composite hardening treatment, in which induction hardening is performed after nitriding or nitrocarburizing treatment, has recently been developed to obtain a thicker hardened layer, thus increasing surface durability and fatigue strength.
With this composite treatment method, if induction hardening is performed after a compound layer such as a nitride film is formed on the surface of an iron and steel material by nitriding treatment, for example, nitrogen contained in the compound layer diffuses over the surface layer part of the iron and steel material, thus increasing a so-called N diffusion layer.
Induction hardening of the above case requires quenching temperature the same as or higher than the Ac1 transformation temperature, at which the material transforms into an austenite structure, and temperature is normally selected from the 750° C. to 1050° C. range. For example, iron and nitrogen are bonded in a nitride layer formed at the nitriding temperature of 570° C. If the nitride layer is reheated up to 650° C. or higher, the layer is decomposed due to oxidation, and the nitrogen in the nitride layer is discharged as nitrogen gas from the outermost surface, whereas the nitrogen is diffused within the layer.
As a result, the surface layer part of the compound layer is oxidized by induction hardening, and the surface of the compound layer turns into an oxidized layer. Since the oxidized layer is porous, whereas the compound layer is dense, the surface of the compound layer is not exposed, being covered with the oxidized layer, or the compound layer may become thinner because at least a part of the layer may be lost. Consequently, mechanical strength, sliding resistance or abrasion resistance inherent in the compound layer may be lost.
Meanwhile, to solve the above problem, namely a nitride layer formed on the surface by nitriding treatment may be damaged or lost due to high-temperature heating if it is subjected to induction hardening as it is, Patent Literature 1 discloses a method in which silicon oxide-based gas nitriding/ion nitriding inhibitor, carburizing inhibitor, anti-oxidant, etc. are applied over the surface having undergone nitriding treatment in thickness of 1 to 3 mm.
In addition, Patent Literature 2 discloses a quenched iron and steel material characterized in that a compound hard nitride layer is formed on the surface of an iron and steel material, and an inorganic compound layer containing at least a type of metal oxide selected from the group of Ti, Zr, Hf, V, Nb, Ta, Cr, W, Mo, and Al is formed as an upper layer of the compound hard nitride layer.